The present invention generally relates to an alkali metal electrochemical cell, and more particularly, to a rechargeable alkali metal cell. Still more particularly, the present invention relates to a lithium ion electrochemical cell activated with an electrolyte having an additive provided to achieve high charge/discharge capacity, long cycle life and to minimize the first cycle irreversible capacity. According to the present invention, the preferred additive to the activating electrolyte is a phosphate compound.
Alkali metal rechargeable cells typically comprise a carbonaceous anode electrode and a lithiated cathode electrode. Due to the high potential of the cathode material (up to 4.3V vs. Li/Li.sup.+ for Li.sub.1-x CoO.sub.2) and the low potential of the carbonaceous anode material (0.01V vs. Li/Li.sup.+ for graphite) in a fully charged lithium ion cell, the choice of the electrolyte solvent system is limited. Since carbonate solvents have high oxidative stability toward typically used lithiated cathode materials and good kinetic stability toward carbonaceous anode materials, they are generally used in lithium ion cell electrolytes. To achieve optimum cell performance (high rate capability and long cycle life), solvent systems containing a mixture of a cyclic carbonate (high dielectric constant solvent) and a linear carbonate (low viscosity solvent) are typically used in commercial secondary cells. Cells with carbonate based electrolytes are known to deliver more than 1000 charge/discharge cycles at room temperature.
U.S. patent application Ser. No. 09/133,799, which is assigned to the assignee of the present invention and incorporated herein by reference, is directed to a quaternary mixture of organic carbonate solvents in the activating electrolyte for a lithium ion cell capable of discharge at temperatures below -20.degree. C. and down to as low as -40.degree. C. while exhibiting good cycling characteristics. The quaternary solvent system includes ethylene carbonate (EC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC) and diethyl carbonate (DEC).
Lithium ion cell design generally involves a trade off in one area for a necessary improvement in another, depending on the targeted cell application. The achievement of a lithium-ion cell capable of low temperature cycleability by use of the above quaternary solvent electrolyte, in place of a typically used binary solvent electrolyte (such as 1.0M LiPF.sub.6 /EC:DMC=30:70, v/v which freezes at -11.degree. C.), is obtained at the expense of increased first cycle irreversible capacity during the initial charging (approximately 65 mAh/g graphite for 1.0M LiPF.sub.6 /EC:DMC:EMC:DEC=45:22:24.8:8.2 vs. 35 mAh/g graphite for 1.0M LiPF.sub.6 /EC:DMC=30:70). Due to the existence of this first cycle irreversible capacity, lithium ion cells are generally cathode limited. Since all of the lithium ions, which shuttle between the anode and the cathode during charging and discharging originally come from the lithiated cathode, the larger the first cycle irreversible capacity, the lower the cell capacity in subsequent cycles and the lower the cell efficiency. Thus, it is desirable to minimize or even eliminate the first cycle irreversible capacity in lithium ion cells while at the same time maintaining the low temperature cycling capability of such cells.
According to the present invention, these objectives are achieved by providing an inorganic or organic phosphate in the quaternary solvent electrolyte. Lithium ion cells activated with these electrolytes exhibit lower first cycle irreversible capacities relative to cells activated with the same quaternary solvent electrolyte devoid of the phosphate additive. As a result, cells including the phosphate additive present higher subsequent cycling capacity than control cells. The cycleability of the present invention cells at room temperature, as well as at low temperatures, i.e., down to about -40.degree. C., is as good as cells activated with the quaternary electrolyte devoid of a phosphate additive.